Stratification in Drying Films Containing Bidisperse Mixtures of Nanoparticles

Large scale molecular dynamics simulations for bidisperse nanoparticle suspensions with an explicit solvent are used to investigate the effects of evaporation rates and volume fractions on the nanoparticle distribution during drying. Our results show that “small-on-top” stratification can occur when Pe_<i>s</i>ϕ_<i>s</i> ≳ <i>c</i> with <i>c</i> ∼ 1, where Pe_<i>s</i> is the Péclet number and ϕ_<i>s</i> is the volume fraction of the smaller particles. This threshold of Pe_<i>s</i>ϕ_<i>s</i> for “small-on-top” is larger by a factor of ∼α² than the prediction of the model treating solvent as an implicit viscous background, where α is the size ratio between the large and small particles. Our simulations further show that when the evaporation rate of the solvent is reduced, the “small-on-top” stratification can be enhanced, which is not predicted by existing theories. This unexpected behavior is explained with thermophoresis associated with a positive gradient of solvent density caused by evaporative cooling at the liquid/vapor interface. For ultrafast evaporation the gradient is large and drives the nanoparticles toward the liquid/vapor interface. This phoretic effect is stronger for larger nanoparticles, and consequently the “small-on-top” stratification becomes more distinct when the evaporation rate is slower (but not too slow such that a uniform distribution of nanoparticles in the drying film is produced), as thermophoresis that favors larger particles on the top is mitigated. A similar effect can lead to “large-on-top” stratification for Pe_<i>s</i>ϕ_<i>s</i> above the threshold when Pe_<i>s</i> is large but ϕ_<i>s</i> is small. Our results reveal the importance of including the solvent explicitly when modeling evaporation-induced particle separation and organization and point to the important role of density gradients brought about by ultrafast evaporation

Stratification in Drying Films Containing Bidisperse Mixtures of Nanoparticles

Large scale molecular dynamics simulations for bidisperse nanoparticle suspensions with an explicit solvent are used to investigate the effects of evaporation rates and volume fractions on the nanoparticle distribution during drying. Our results show that “small-on-top” stratification can occur when Pe_<i>s</i>ϕ_<i>s</i> ≳ <i>c</i> with <i>c</i> ∼ 1, where Pe_<i>s</i> is the Péclet number and ϕ_<i>s</i> is the volume fraction of the smaller particles. This threshold of Pe_<i>s</i>ϕ_<i>s</i> for “small-on-top” is larger by a factor of ∼α² than the prediction of the model treating solvent as an implicit viscous background, where α is the size ratio between the large and small particles. Our simulations further show that when the evaporation rate of the solvent is reduced, the “small-on-top” stratification can be enhanced, which is not predicted by existing theories. This unexpected behavior is explained with thermophoresis associated with a positive gradient of solvent density caused by evaporative cooling at the liquid/vapor interface. For ultrafast evaporation the gradient is large and drives the nanoparticles toward the liquid/vapor interface. This phoretic effect is stronger for larger nanoparticles, and consequently the “small-on-top” stratification becomes more distinct when the evaporation rate is slower (but not too slow such that a uniform distribution of nanoparticles in the drying film is produced), as thermophoresis that favors larger particles on the top is mitigated. A similar effect can lead to “large-on-top” stratification for Pe_<i>s</i>ϕ_<i>s</i> above the threshold when Pe_<i>s</i> is large but ϕ_<i>s</i> is small. Our results reveal the importance of including the solvent explicitly when modeling evaporation-induced particle separation and organization and point to the important role of density gradients brought about by ultrafast evaporation

Structure and Strength at Immiscible Polymer Interfaces

Thermal welding of polymer–polymer interfaces is important for integrating polymeric elements into devices. When two different polymers are joined, the strength of the weld depends critically on the degree of immiscibility. We perform large-scale molecular dynamics simulations of the structure–strength relation at immiscible polymer interfaces. Our simulations show that immiscibility arrests interdiffusion and limits the equilibrium interfacial width. Even for weakly immiscible films, the narrow interface is unable to transfer stress upon deformation as effectively as the bulk material, and chain pullout at the interface becomes the dominant failure mechanism. This greatly reduces the interfacial strength. The weak response of immiscible interfaces is shown to arise from an insufficient density of entanglements across the interface. We demonstrate that there is a threshold interfacial width below which no significant entanglements can form between opposite sides to strengthen the interface

Tension Amplification in Tethered Layers of Bottle-Brush Polymers

Molecular dynamics simulations of a coarse-grained bead–spring model have been used to study the effects of molecular crowding on the accumulation of tension in the backbone of bottle-brush polymers tethered to a flat substrate. The number of bottle-brushes per unit surface area, Σ, as well as the lengths of the bottle-brush backbones <i>N</i>_bb (50 ≤ <i>N</i>_bb ≤ 200) and side chains <i>N</i>_sc (50 ≤ <i>N</i>_sc ≤ 200) were varied to determine how the dimensions and degree of crowding of bottle-brushes give rise to bond tension amplification along the backbone, especially near the substrate. From these simulations, we have identified three separate regimes of tension. For low Σ, the tension is due solely to intramolecular interactions and is dominated by the side chain repulsion that governs the lateral brush dimensions. With increasing Σ, the interactions between bottle-brush polymers induce compression of the side chains, transmitting increasing tension to the backbone. For large Σ, intermolecular side chain repulsion increases, forcing side chain extension and reorientation in the direction normal to the surface and transmitting considerable tension to the backbone

Effects of Functional Groups and Ionization on the Structure of Alkanethiol-Coated Gold Nanoparticles

We report classical atomistic molecular dynamics simulations of alkanethiol-coated gold nanoparticles solvated in water and decane, as well as at water/vapor interfaces. The structure of the coatings is analyzed as a function of various functional end groups, including amine and carboxyl groups in various ionization states. We study both neutral and charged end groups for two different chain lengths (9 and 17 carbons). For the charged end groups, we simulated both mono- and divalent counterions. For the longer alkanes, we find significant local bundling of chains on the nanoparticle surface, which results in highly asymmetric coatings. In general, the charged end groups attenuate this effect by enhancing the water solubility of the nanoparticles. On the basis of the coating structures and density profiles, we can qualitatively infer the overall solubility of the nanoparticles. This asymmetry in the alkanethiol coatings is likely to have a significant effect on aggregation behavior. Our simulations elucidate the mechanism by which modulating the end group charge state can be used to control coating structure and therefore nanoparticle solubility and aggregation behavior

Structured Ionomer Thin Films at Water Interface: Molecular Dynamics Simulation Insight

Controlling the structure and dynamics of thin films of ionizable polymers at water interfaces is critical to their many applications. As the chemical diversity within one polymer is increased, controlling the structure and dynamics of the polymer, which is a key to their use, becomes a challenge. Here molecular dynamics simulations (MD) are used to obtain molecular insight into the structure and dynamics of thin films of one such macromolecule at the interface with water. The polymer consists of an ABCBA topology with randomly sulfonated polystyrene (C), tethered symmetrically to flexible poly­(ethylene-<i>r</i>-propylene) blocks (B), and end-capped by a poly­(<i>t</i>-butylstyrene) block (A). The compositions of the interfacial and bulk regions of thin films of the ABCBA polymers are followed as a function of exposure time to water. We find that interfacial rearrangements take place where buried ionic segments migrate toward the water interface. The hydrophobic blocks collapse and rearrange to minimize their exposure to water. The water that initially drives interfacial reengagements breaks the ionic clusters within the film, forming a dynamic hydrophilic internal network within the hydrophobic segments

Structure and Dynamics of Ionic Block Copolymer Melts: Computational Study

Structure and dynamics of melts of copolymers with an ABCBA topology, where C is an ionizable block, have been studied by fully atomistic molecular dynamics (MD) simulations. Introducing an ionizable block for functionality adds a significant element to the coupled set of interactions that determine the structure and dynamics of the macromolecule. The polymer consists of a randomly sulfonated polystyrene C block tethered to a flexible poly­(ethylene-<i>r</i>-propylene) bridge B and end-capped with poly­(<i>tert</i>-butylstyrene) A. The chemical structure and topology of these polymers constitute a model for incorporation of ionic blocks within a framework that provides tactility and mechanical stability. Here we resolve the structure and dynamics of a structured polymer on the nanoscale constrained by ionic clusters. We find that the melts form intertwined networks of the A and C blocks independent of the degree of sulfonation of the C block with no long-range order. The cluster cohesiveness and morphology affect both macroscopic translational motion and segmental dynamics of all the blocks

Internal Correlations and Stability of Polydots, Soft Conjugated Polymeric Nanoparticles

Conjugated polymers collapsed into long-lived highly luminescent nanoparticles, or polydots, have opened a new paradigm of tunable organic particles with an immense potential enhancing intracellular imaging and drug delivery. Albeit the chains are not in their equilibrium conformation and are not confined by cross-links, they remain stable over astounding long times. Using fully atomistic molecular dynamics simulations with an innovative method to controllably collapse an inherently rigid polymer, we determined for the first time the internal structure and stability of polydots made of dialkyl-<i>para</i>-phenylene ethynylene, immersed in water, a biological relevant medium. In contrast to natural aggregates, the aromatic rings within the polydots are uncorrelated, with little to no water in its interior. This lack of correlation explains the differences of luminescence characteristics between spontaneously aggregated conjugated polymers and polydots. Resolving the conformation and stability of these particles will enable transforming an idea to a new effective tool

Coarse-Graining Atactic Polystyrene and Its Analogues

Capturing large length scales in polymers and soft matter while retaining atomistic properties is imperative to computational studies of dynamic systems. Here we present the results for a coarse-grained model based on atomistic simulation of atactic polystyrene (PS). Similar to previous work by Harmandaris et al. and Fritz et al., each monomer is described by two coarse-grained beads. In contrast to these early studies in which intramolecular potentials were based on Monte Carlo simulations of isotactic and syndiotactic single PS molecules to capture stereochemistry, we obtained intramolecular interactions from a single molecular dynamics simulation of an all-atom atactic PS melt. The nonbonded interactions are obtained using the iterative Boltzmann inversion (IBI) scheme. This methodology has been extended to coarse graining of poly­(4-<i>tert</i>-butylstyrene) (PtBS) in which an additional type of coarse-grained bead is used to describe the <i>tert</i>-butyl group. Similar to the process for PS, the intramolecular interactions are obtained from a single all-atom melt simulation for atactic PtBS

Polymers at Liquid/Vapor Interface

Polymers confined to the liquid/vapor interface are studied using molecular dynamics simulations. We show that for polymers which are weakly immiscible with the solvent, the density profile perpendicular to the liquid/vapor interface is strongly asymmetric. On the vapor side of the interface, the density distribution falls off as a Gaussian with a decay length on the order of the bead diameter, whereas on the liquid side, the density profile decays as a simple exponential. This result differs from that of a polymer absorbed from a good solvent with the density profile decaying as a power law. As the surface coverage increases, the average end-to-end distance and chain mobility systematically decreases toward that of the homopolymer melt